In the course of drilling oil and gas wells and the like by means of a rotary well drilling apparatus, drilling fluid or drilling mud is pumped down through a rotary drill pipe and discharged at or adjacent the bit at the lower end thereof. In normal drilling operations, a portion of this circulation fluid returns upward through the well bore and is conducted to a sump where it is filtered and returned to the pump and again pumped down the drill pipe.
The primary functions of the drilling fluid or drilling mud are: (1) to lubricate and cool the drilling bit; (2) to suspend the cuttings from the drilling operations so that they are carried to the surface and removed; (3) to prevent excessive amounts of fluids from flowing from the well bore into surrounding formations by depositing on the wall of the hole a thin, but substantial, impervious filter cake; (4) to serve as a weighting material exerting sufficient pressure to counterbalance any pressure exerted by water, gas, oil, or other earth fluid, and (5) to prevent caving or other intrusions into the drill hole.
Thus, an efficient drilling mud must exhibit numerous characteristics, including, for example, viscosity, initial and 10-minute gel strength, thixotropy, fluid loss prevention, stability under various temperature and pressure operating conditions, stability against contaminating fluids, such as salt water, calcium sulfate, cement and potassium ion contaminated fluids, etc.
Aqueous clay-based drilling fluids or muds are well-known in the prior art, as are different additives. Such fluids are comprised basically of water, a clay such as bentonite or sepiolite, lignosulfonate, a base such as NaOH, a densifier such as barite or barium sulfate, and possibly a salt containing a cation such as sodium or potassium. Other aqueous ionic compounds such as Na.sup.+ Cl.sup.- may also be present. These fluids are suitable for drilling at pH levels from about 8 to about 11.5.
Oil-producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water, or gas. Consequently, the rate of oil production is largely determined by the rate of flow through these permeable formations which, in turn, is dependent upon the porosity or permeability of the sand or stone present. In drilling through such a porous layer, it is desirable to employ a drilling fluid having such characteristics that excessive amounts of liquids or solids are prevented from penetrating through the porous formation. The ability of the drilling fluid to prevent excessive formation fluid penetration is called filtration control.
Materials that have been used in the past to control filtration rates of aqueous drilling fluids by plugging, producing cakes, or similar methods, have included materials such as pregelatinized starch, sodium carboxymethylcellulose, sodium polyacrylates, and lignites. Each of these materials have certain limitations. For example, lignite becomes ineffective in high salt concentrations. A drilling fluid containing a copolymer filtration agent is disclosed in U.S. Pat. No. 4,547,299 which issued to Lucas on Oct. 15, 1985. Additive systems for controlling fluid loss in aqueous drilling fluids at high temperatures are discussed in U.S. Pat. No. 4,626,362 which issued to Dickert et al. on Dec. 2, 1986.
In drilling deep wells, bottom-hole temperatures over 300.degree. F. are often encountered which cause many conventional drilling fluid additives to thermally degrade. As additives degrade, essential drilling fluid properties, such as filtration control and rheology, also deteriorate. Continual addition of fresh additives to maintain fluid properties can be costly.
Therefore, what is needed is fluid loss additive for a water-based drilling fluid which improves filtration control and which remains stable for extended periods at deep well drilling temperatures.